Cryogenic refrigerator

ABSTRACT

In a cryogenic refrigerator, a displacer includes a low temperature end and a high temperature end. A cylinder accommodates the displacer to be reciprocated in a longitudinal direction, and forms an expansion space which is an expansion space of a refrigerant gas between the low temperature end of the second displacer and the cylinder. The damper portion is disposed to be adjacent to the expansion space and communicates with the expansion space. A volume of the damper portion is 30% or less of a volume of the expansion space.

RELATED APPLICATIONS

Priority is claimed to Japanese Patent Application No. 2014-027654, filed Feb. 17, 2014, the entire content of which is incorporated herein by reference.

BACKGROUND

1. Technical Field

Certain embodiments of the invention relate to a cryogenic refrigerator which cools a cooling object using cooling generated by performing Simon expansion on high pressure refrigerant gas supplied from a compressor.

2. Description of Related Art

In a displacer-type cryogenic refrigerator, a displacer reciprocates in a cylinder, a refrigerant gas in an expansion space expands, and thus, cooling is generated. In addition, in a pulse tube-type cryogenic refrigerator, a gas piston reciprocates in a pulse tube, a refrigerant gas in an expansion space expands, and thus, cooling is generated. The cooling of the refrigerant gas generated in the expansion space is transmitted to a cooling stage while being accumulated in a refrigerator and cools a cooling object which is connected to the cooling stage. In addition, for example, helium gas is used as the refrigerant gas.

SUMMARY

According to an embodiment of the present invention, there is provided a cryogenic refrigerator including: a displacer which includes a low temperature end and a high temperature end; a cylinder which accommodates the displacer to be reciprocated in a longitudinal direction and forms an expansion space of a refrigerant gas between the low temperature end of the displacer and the cylinder; and a damper portion which is disposed to be adjacent to the expansion space and communicates with the expansion space. A volume of the damper portion is 30% or less of a volume of the expansion space.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically showing an example of a cryogenic refrigerator according to an embodiment.

FIG. 2 is a diagram showing a temperature change of a specific heat in cryogenic temperature regions of helium and various metal materials.

FIGS. 3A to 3C are diagrams showing a damper portion according to the embodiment.

FIG. 4 is a diagram showing test results, in which an average temperature and a temperature vibration width of a second cooling stage with respect to the volume of the damper portion are measured, in a table form.

FIG. 5 is a diagram showing an instantaneous change in the temperature of the second cooling stage.

FIG. 6 is a diagram schematically showing a pulse tube-type cryogenic refrigerator.

DETAILED DESCRIPTION

For example, a cryogenic refrigerator is used as a cooling device of a superconducting magnet in a Magnetic Resonance Imaging (MRI) which is used to generate a diagnostic image in a medical field or the like, or a Nuclear Magnetic Resonance (NMR). Moreover, the cryogenic refrigerator is also used to cool a Superconducting Quantum Interference Device (SQUID) element, an infrared sensor, or the like. Accordingly, if temperature fluctuation occurs in cooling generated by the cryogenic refrigerator, a performance decrement or a noise increase of diagnostic equipment or a sensor may occur due to the unstable temperature. Moreover, in order to maintain the performance of the diagnostic equipment or the sensor, an average temperature of the cooling generated by the cryogenic refrigerator also should be maintained at a low state.

It is desirable to decrease temperature amplitude while maintaining an average temperature of the cooling generated by the cryogenic refrigerator.

Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

First, with reference to FIG. 1, an outline of a cryogenic refrigerator 1 according to the embodiment will be described. FIG. 1 is a schematic diagram showing an example of the cryogenic refrigerator 1 according to the embodiment. For example, the cryogenic refrigerator 1 according to the embodiment is a Gifford-McMahon (GM) type cryogenic refrigerator using helium gas as a refrigerant gas. As shown in FIG. 1, the cryogenic refrigerator 1 includes a first displacer 2, and a second displacer 3 which is connected to the first displacer 2 in a longitudinal direction. For example, the first displacer 2 and the second displacer 3 are connected to each other via a pin 4, a connector 5, and a pin 6.

A first cylinder 7 and a second cylinder 8 are integrally formed with each other, and respectively include a high temperature end and a low temperature end. The low temperature end of the first cylinder 7 and the high temperature end of the second cylinder 8 are connected to each other at a bottom portion of the first cylinder 7. The second cylinder 8 is formed so as to coaxially extend with the first cylinder 7 and is a cylinder member having a smaller diameter than the first cylinder 7. The first cylinder 7 is a container which accommodates the first displacer 2 to be reciprocated in the longitudinal direction. In addition, the second cylinder 8 is a container which accommodates the second displacer 3 to be reciprocated in the longitudinal direction.

Considering strength, thermal conductivity, helium shielding capability, or the like, for example, the first cylinder 7 and the second cylinder 8 are formed of stainless steel. The outer peripheral portion of the second displacer 3 is configured of a pipe formed of metal such as stainless steel. A film of an abrasion-resistant resin such as a fluorocarbon resin may be formed on the outer peripheral surface of the second displacer 3.

A scotch yoke mechanism (not illustrated) which reciprocates the first displacer 2 and the second displacer 3 is provided on the high temperature end of the first cylinder 7. The first displacer 2 and the second displacer 3 respectively reciprocate along the first cylinder 7 and the second cylinder 8. The first displacer 2 and the second displacer 3 respectively include a high temperature end and a low temperature end.

The first displacer 2 has a cylindrical outer peripheral surface, and the inner portion of the first displacer 2 is filled with a first regenerator material. An internal volume of the first displacer 2 functions as a first regenerator 9. A straightening device 10 is installed on the upper portion of the first regenerator 9, and a straightening device 11 is installed on the lower portion of the first regenerator 9. A first opening 13, through which the refrigerant gas circulates from a room temperature chamber 12 to the first displacer 2, is formed on the high temperature end of the first displacer 2.

The room temperature chamber 12 is a space which is formed of the first cylinder 7 and the high temperature end of the first displacer 2. The volume of the room temperature chamber 12 is changed because of the reciprocation of the first displacer 2. In pipes which connect suction and discharge systems configured of the compressor 14, a supply valve 15, and a return valve 16 to each other, a supply-exhaust common pipe is connected to the room temperature chamber 12. In addition, a seal 17 is mounted between a portion on the high temperature end of the first displacer 2 and the first cylinder 7.

A second opening 19, through which the refrigerant gas is introduced into a first expansion space 18 via a first clearance C1, is formed on the low temperature end of the first displacer 2. The first expansion space 18 is a space which is formed of the first cylinder 7 and the first displacer 2. The volume of the first expansion space 18 is changed because of the reciprocation of the first displacer 2. In the outer periphery of the first cylinder 7, a first cooling stage 20 which is thermally connected to a cooling object (not shown) is disposed at a position corresponding to the first expansion space 18. The first cooling stage 20 is cooled by the refrigerant gas passing through the first clearance C1.

The second displacer 3 has a cylindrical outer peripheral surface. The inner portion of the second displacer 3 is divided into two stages in the axial direction, in which an upper end straightening device 21 and a lower end straightening device 22 are provided, and a retainer 23 positioned approximately at the center in the vertical direction is interposed between the straightening devices 21 and 22. In the internal volume of the second displacer 3, for example, a high temperature side region 24, which is positioned at a higher temperature side than the retainer 23, is filled with a second regenerator material formed of a nonmagnetic material such as lead or bismuth. For example, a low temperature side region 25 which is positioned at the lower temperature side (the lower end) of the retainer 23 is filled with a third regenerator material formed of a magnetic material such as HoCu₂ which is different from the regenerator material of the high temperature side region 24. The lead, the bismuth, HoCu₂, or the like are spherically formed, a plurality of spherical formations are collected, and thus, the regenerator material is configured. The retainer 23 prevents the regenerator material of the high temperature side region 24 and the regenerator material of the low temperature side region 25 from being mixed with each other. The high temperature side region 24 and the low temperature side region 25 which make up the internal volume of the second displacer 3 function as a second regenerator 34. The first expansion space 18 and the high temperature end of the second displacer 3 communicate with each other through a communication path around the connector 5. The refrigerant gas circulates from the first expansion space 18 to the second regenerator 34 via the communication path.

A third opening 27 for circulating the refrigerant gas into a second expansion space 26 via a second clearance C2 is formed on the low temperature end of the second displacer 3. The second expansion space 26 is a space which is formed of the second cylinder 8 and the second displacer 3. The volume of the second expansion space 26 is changed because of the reciprocation of the second displacer 3. The second clearance C2 is formed of the low temperature end of the second cylinder 8 and the second displacer 3.

A second cooling stage 28 which is thermally connected to the cooling object is disposed at a position corresponding to the second expansion space 26 of the outer periphery of the second cylinder 8. The second cooling stage 28 is cooled by the refrigerant gas passing through the second clearance C2.

For example, from the viewpoint of specific gravity, strength, thermal conductivity, or the like, the first displacer 2 is formed of a fabric based phenol resin or the like. For example, the first regenerator material is configured of a wire screen or the like. In addition, for example, the second displacer 3 is configured by interposing the spherical second regenerator material such as lead or bismuth in the axial direction by a felt and wire screen. Moreover, as described above, the internal volume of the second displacer 3 may be divided into a plurality of areas by the retainers.

The first displacer 2 and the second displacer 3 may include a cover portion 29 and a cover portion 30 on the low temperature end, respectively. From the viewpoint of joining a displacer main body, the cover portion 29 and the cover portion 30 have a two-stage columnar shape. The cover portion 29 is fixed to the first displacer 2 by a press-fit pin 31, and the cover portion 30 is fixed to the second displacer 3 by a press-fit pin 32.

Next, an operation of the cryogenic refrigerator 1 according to the embodiment will be described. At a certain point of time in a refrigerant gas supply process, the first displacer 2 and the second displacer 3 are positioned at bottom dead centers of the first cylinder 7 and the second cylinder 8. If the supply valve 15 is opened at the same time as that the certain point of time, or at timing slightly deviated from that the certain point of time, a high pressure helium gas (for example, helium gas of 2.2 MPa) is supplied from the supply-exhaust common pipe into the first cylinder 7 through the supply valve 15, and the high pressure helium gas flows from the first opening 13 positioned at the upper portion of the first displacer 2 into the first regenerator 9 positioned in the first displacer 2. The high pressure helium gas flowing into the first regenerator 9 is supplied to the first expansion space 18 via the second opening 19 and the first clearance Cl positioned at the lower portion of the first displacer 2 while being cooled by the first regenerator material.

The high pressure helium gas supplied to the first expansion space 18 flows into the second regenerator 34 inside the second displacer 3 via a communication path around the connector 5. The high pressure helium gas flowing into the second regenerator 34 is supplied to the second expansion space 26 via the third opening 27 and the second clearance positioned at the lower portion of the second displacer 3 while being cooled by the second regenerator material.

In this way, the first expansion space 18 and the second expansion space 26 are filled with the high pressure helium gas, and the supply valve 15 is closed. In this case, the first displacer 2 and the second displacer 3 are positioned at top dead centers in the first cylinder 7 and the second cylinder 8. If the return valve 16 is opened at the same time as that, or at timing slightly deviated from that, the refrigerant gas in the first expansion space 18 and the second expansion space 26 is expanded, and becomes a low pressure helium gas (for example, helium gas of 0.8 MPa). In this case, cooling is generated by the expansion of the refrigerant gas. The helium gas in the first expansion space 18, which becomes a low temperature by the expansion, absorbs heat of the first cooling stage 20 via the first clearance C1. In addition, the helium gas in the second expansion space 26 absorbs heat of the second cooling stage 28 via the second clearance C2.

The first displacer 2 and the second displacer 3 move toward the bottom dead centers, and thus, the volumes of the first expansion space 18 and the second expansion space 26 decrease. The helium gas in the second expansion space 26 is returned to the first expansion space 18 via the second clearance C2, the third opening 27, the second regenerator 34, and the communication path. In addition, the helium gas in the first expansion space 18 is returned to the suction side of the compressor 14 via the second opening 19, the first regenerator 9, and the first opening 13. At this time, the first regenerator material, the second regenerator material, and the third regenerator material are cooled by the refrigerant gas. That is, the first regenerator material, the second regenerator material, and the third regenerator material accumulate the cooling generated by the expansion of the refrigerant gas. This process is set to one cycle, the first cooling stage 20 and the second cooling stage 28 is cooled by repeating the cooling cycle. In addition, the frequency of the cooling cycle of the cryogenic refrigerator 1 is several hertz to several tens of hertz.

Hereinbefore, the outline of the cryogenic refrigerator 1 according to the embodiment is described with reference to FIG. 1. In the above, for simple description, descriptions with respect to a damper portion included in the cryogenic refrigerator 1 according to the embodiment are omitted. Hereinafter, the damper portion included in the cryogenic refrigerator 1 according to the embodiment will be described in detail.

As described above, the cryogenic refrigerator 1 cools the first cooling stage 20 and the second cooling stage 28 by repeating the cooling cycle. In the first cooling stage 20 and the second cooling stage 28, the second cooling stage 28 reaches a cryogenic temperature of approximately 4 K. Accordingly, when the cryogenic refrigerator 1 is used to cool a superconducting magnet or various sensors, the second cooling stage 28 comes into contact with an electronic device which is the cooling object.

During the cooling cycle of the cryogenic refrigerator 1, the second cooling stage 28 is cooled by the cooling which is generated by the refrigerant gas expanding in the second expansion space 26. Accordingly, the temperature of the second cooling stage 28 is periodically changed according to the cooling cycle of the cryogenic refrigerator 1. As a result, the temperature of the electronic device which comes into thermal contact with the second cooling stage 28 is periodically changed. The change of the temperature becomes a cause of the performance decrement of the electronic device.

Accordingly, the cryogenic refrigerator 1 according to the embodiment includes the damper portion which is adjacent to the second expansion space 26 and communicates with the second expansion space 26. In order to communicate with the second expansion space 26, the damper portion shares the refrigerant gas with the second expansion space 26. The refrigerant gas existing in the damper portion functions as a so-called buffer material without a thermal buffer, which decreases the temperature change of the refrigerant gas in the second expansion space 26. Hereinafter, a configuration and an operation of the damper portion will be described. Moreover, when the cryogenic refrigerator 1 is a multistage cryogenic refrigerator which includes the displacer having three stages or more, the damper portion may be provided to be adjacent to the expansion space which is formed between the lowest temperature side displacer and the cylinder.

FIG. 2 is a diagram showing the temperature changes of the specific heat in cryogenic temperature regions of helium and various metal materials. More specifically, FIG. 2 is a graph in which the temperatures of the metal material and the helium are plotted on a horizontal axis and the specific heat of the metal material is plotted on a vertical axis. As shown in FIG. 2, the specific heat of the metal material such as copper is abruptly decreased in the temperature region of the cryogenic temperature of 20 K or less. Meanwhile, in this temperature region, the specific heat of the helium is higher than the specific heat of the metal material.

FIGS. 3A to 3C are diagrams showing a damper portion 35 according to the embodiment, and are diagrams showing the second cylinder 8 of FIG. 1 and the periphery thereof in an enlarged manner. As shown in FIGS. 3A to 3C, the damper portion 35 accommodating the helium gas which is the refrigerant gas is provided to be adjacent to the second expansion space 26, and thus, the volume of the second expansion space 26 increases. The helium gas is collected in the damper portion 35, and thus, heat capacity of a cooling portion including the second expansion space 26 and the damper portion increases. As a result, the temperature amplitude of the second cooling stage 28 also decreases.

FIG. 3A shows the case where the damper portion 35 is provided in the second displacer 3. More specifically, in FIG. 3A, the damper portion 35 is provided on the cover portion 30 included in the low temperature end of the second displacer 3. As shown in FIG. 3A, the damper portion 35 has a short pipe shape in which one end is opened and the other end is closed. Here, one end of the damper portion 35 is opened in a direction adjacent to the second expansion space 26. Moreover, the other end of the damper portion 35 is closed by the cover portion 30 of the second displacer 3. In addition, FIG. 1 shows the cryogenic refrigerator 1 including the damper portion 35 shown in FIG. 3A.

The damper portion 35 shown in FIG. 3A reciprocates because of the reciprocation of the second displacer 3. When the second displacer 3 is positioned at the bottom dead center, the volume of the second expansion space 26 becomes approximately 0. Meanwhile, even when the second displacer 3 is positioned at the bottom dead center, the damper portion 35 can collect the helium gas.

As shown in FIG. 3A, the one end of the damper portion 35 is largely opened toward the second expansion space 26. Accordingly, the passage resistance of the helium gas between the second expansion space 26 and the damper portion 35 decreases. Therefore, the helium gas between the second expansion space 26 and the damper portion 35 can rapidly circulate in a short amount of time. Accordingly, the temperature difference between the helium gas in the second expansion space 26 and the helium gas in the damper portion 35 decreases. This also is the reason why it is possible to prevent the temperature change of the second cooling stage 28.

In some cases, the cryogenic refrigerator 1 is used as a portion of another measurement device, and an increase in the size of cryogenic refrigerator 1, particularly, an increase in the entire length thereof is not preferable. As shown in FIG. 3A, the damper portion 35 is provided on the cover portion 30 of the lower temperature end side of the second displacer 3, which exists in the related art, and thus, the size of the cryogenic refrigerator 1 is not changed even when the damper portion 35 is added. In addition, even when the size is changed, it is possible to decrease the width of the increase.

FIG. 3B shows the case where the damper portion 35 is provided in the second cylinder 8. As shown in FIG. 3B, the second cooling stage 28 is provided on the bottom portion and the outer periphery of the second cylinder 8 at the position corresponding to the second expansion space 26. The second cooling stage 28 is a flanged member which has a protruding portion 36 extending in the direction perpendicular to the long axis of the second cylinder 8. The damper portion 35 shown in FIG. 3B is provided in the protruding portion 36 of the second cooling stage 28.

For example, the second cooling stage 28 is formed of a metal having high thermal conductivity such as copper. In the second cooling stage 28 shown in FIG. 3A, the locations corresponding to the bottom portion and the outer periphery of the second cylinder 8 are filled with a metal. Meanwhile, in the second cooling stage 28 shown in FIG. 3B, cavities are provided in the locations corresponding to the bottom portion and the outer periphery of the second cylinder 8, and the helium gas can be collected at the locations.

In FIG. 3B, a dashed line indicted by a reference numeral 30′ shows the position of the cover portion 30 when the second displacer 3 is positioned at the bottom dead center. As shown in FIG. 3B, even when the second displacer 3 is positioned at the bottom dead center, it is possible to collect the helium gas in the damper portion 35 provided in the protruding portion 36 of the second cooling stage 28. Accordingly, the damper portion 35 shown in FIG. 3B has a short hollow pipe shape in which the cross section is a ring shape. The side surface of the inner diameter side is opened toward the second expansion space 26, and the other surface is closed by the outer peripheral portion of the second cooling stage 28.

Similar to the damper portion 35 shown in FIG. 3A, also in the damper portion 35 shown in FIG. 3B, the second cooling stage 28 existing in the related art is provided, and thus, the size of the cryogenic refrigerator 1 is not changed even when the damper portion 35 is added. Even when the damper portion 35 is provided and the size of the cryogenic refrigerator 1 is changed, it is possible to decrease the width of the size increase.

Similar to the FIG. 3B, FIG. 3C shows the case when the damper portion 35 is provided in the second cylinder 8. In the example shown in FIG. 3C, the shape of the second cooling stage 28 is similar to the example shown in FIG. 3B. That is, in the second cooling stage 28, cavities are provided at the locations corresponding to the bottom portion and the outer periphery of the second cylinder 8. Here, the difference between the example shown in FIG. 3B and the example shown in FIG. 3C is the position of the cover portion 30 when the second displacer 3 is positioned at the bottom dead center.

Also in the example shown in FIG. 3C, the dashed line indicated by the reference numeral 30′ shows the position of the cover portion 30 when the second displacer 3 is positioned at the bottom dead center. In the example shown in FIG. 3C, even when the second displacer 3 is positioned at the bottom dead center, the second displacer 8 does not reach the cavity provided in the inner portion of the second cylinder 8. Accordingly, in the example shown in FIG. 3C, the space functions as the damper portion 35, in which the space in the protruding portion 36 of the second cooling stage 28 and the space provided in the position corresponding to the bottom portion of the second cylinder 8 in the second cooling stage 28 are combined with each other.

Accordingly, in the example shown in FIG. 3C, the damper portion 35 has a short pipe shape, and an opening is provided on a portion of one end which comes into contact with the second expansion space 26. The opening portion is opened toward the second expansion space 26. In addition, the other end of the damper portion 35 is closed by the outer peripheral portion of the second cooling stage 28. The effects of the damper portion 35 shown in FIG. 3C are similar to the above-described effects of the damper portion 35 shown in FIG. 3B. In addition, as the damper portion 35, the shape shown in FIG. 3A and shapes shown in FIGS. 3B and 3C may be used together.

As described above, the damper portion 35 is provided in the cryogenic refrigerator 1, and thus, it is possible to decrease the amplitude of the temperature change in the second cooling stage 28. Here, the increase in the helium gas accommodated in the damper portion 35, that is, the increase in the volume of the damper portion 35 can decrease the amplitude of the temperature change in the second cooling stage 28.

Here, the helium gas flowing in the second cylinder 8 increases as the volume of the damper portion 35 increases. As a result, the helium gas in the first cylinder 7 decreases. The pressure difference of the helium gas in the first cylinder 7 decreases as the helium gas in the first cylinder 7 decreases. According to this, the cooling generated in the first expansion space 18 decreases, and thus, the temperature of the low temperature end of the first cylinder 7 increases. That is, according to the influence due to the increase in the volume of the damper portion 35, the first cylinder 7 is more sensitive than the second cylinder 8, and the influence is represented in the first cylinder 7 before the second cylinder 8. If the volume of the damper portion 35 increases, first, the temperature of the low temperature end of the first cylinder 7 increases.

As described above, the low temperature end of the first cylinder 7 and the high temperature end of the second cylinder 8 are connected to each other at the bottom portion of the first cylinder 7. Accordingly, the increase in the temperature of the low temperature end of the first cylinder 7 means the increase in the temperature of the high temperature end of the second cylinder 8. Finally, the temperature of the low temperature end of the second cylinder 8 also increases.

Accordingly, even when it is possible to decrease the amplitude of the temperature change in the second cooling stage 28, if the average temperature of the second cooling stage 28 is increased, it is not possible to achieve the original purpose of cooling the cooling object, and thus, the decrease in the amplitude of the temperature change becomes meaningless. Accordingly, the volume of the damper portion 35 cannot increase without limitation.

FIG. 4 is a diagram showing test results, in which the average temperature and the temperature vibration width of the second cooling stage 28 with respect to the volume of the damper portion 35 are measured, in a table form. In FIG. 4, a “damper volume/expansion chamber volume” indicates a ratio of the volume of the damper portion 35 with respect to the volume of the second expansion space 26. Here, the “volume of the second expansion space 26” means the difference between the volume when the second displacer 3 is positioned at the top dead center and the volume when the second displacer 3 is positioned at the bottom dead center. This volume also means the volume of the region in which the low temperature end of the second displacer 3 moves in the second cylinder 8.

In FIG. 4, a “two-stage temperature” indicates the average value (average temperature) of the temperature change of the second cooling stage 28, and the unit is K (Kelvin). In addition, a “temperature vibration width” indicates the amplitude of the temperature change of the second cooling stage 28, and the unit is mK (mili-Kelvin).

As shown in FIG. 4, the inventors confirmed that the average temperature of the second cooling stage 28 was not increased even when the volume of the damper portion 35 was approximately ¼ (0.23) of the volume of the second expansion space 26, through testing. Similarly, the inventors confirmed that the amplitude of the temperature change of the second cooling stage 28 decreased as the volume of the damper portion 35 with respect to the volume of the second expansion space 26 increased, through testing.

The inventors assumed that the average temperature of the second cooling stage 28 was not increased until the volume of the damper portion 35 became approximately ⅓ (35%) of the volume of the second expansion space 26 or the average temperature was within an allowable range even if the average temperature was increased. According to the examination of the inventors, preferably, the volume of the damper portion 35 is approximately 30% or less of the volume of the second expansion space 26. In addition, as shown in FIG. 4, even when the volume of the damper portion 35 is approximately 10% (0.12) of the volume of the second expansion space 26, an effect is exerted in which the amplitude of the temperature change of the second cooling stage 28 is decreased. If the damper portion 35 can collect the helium gas even a little, in principle, the effect of suppressing the amplitude of the temperature change of the second cooling stage 28 can be obtained. According to the examination of the inventors, if the volume of the damper portion 35 is 5% or more of the volume of the damper portion 35, preferably, 10% or more, it is possible to effectively suppress the amplitude of the temperature change of the second cooling stage 28.

FIG. 5 is a diagram showing a instantaneous change in the temperature of the second cooling stage 28. FIG. 5 is a graph in which the time is plotted on the horizontal axis and the temperature of the second cooling stage 28 is plotted on the vertical axis. In FIG. 5, a graph indicated by a dashed line shows the instantaneous change in the temperature of the second cooling stage 28 in the cryogenic refrigerator 1 in the related art in which the damper portion 35 is not included. This corresponds to Test A in FIG. 4. Meanwhile, in FIG. 5, a graph indicated by a solid line shows the instantaneous change in the temperature of the second cooling stage 28 in the cryogenic refrigerator 1 in which the damper portion 35 is included at the position shown in FIG. 3A. Here, the volume of the damper portion 35 is 0.353 cc, and corresponds to Test B in FIG. 4.

As shown in FIG. 5, the damper portion 35 is provided in the cryogenic refrigerator 1 of the related art, and thus, it is possible to suppress the amplitude of the temperature change of the second cooling stage 28. Specifically, comparing the temperature change of the second cooling stage 28 in the cryogenic refrigerator 1 of the related art indicated by the dashed line in FIG. 5 and the temperature change of the second cooling stage 28 in the cryogenic refrigerator 1 according to the embodiment indicated by the solid line, the lowest attainable temperature of the former is lower than the lowest attainable temperature of the latter, and the highest attainable temperature of the former is higher than the highest attainable temperature of the latter. As shown in FIG. 5, even when the damper portion 35 is provided in the cryogenic refrigerator 1, the average temperature of the second cooling stage 28, that is, the center of the amplitude is not substantially changed. In addition, the lowest attainable temperature of the cryogenic refrigerator 1 of the related art is lower than that of cryogenic refrigerator 1 according to the embodiment, and this is obvious because the temperature amplitude is decreased while the average temperature is maintained in the cryogenic refrigerator 1 according to the embodiment.

Hereinbefore, the aspect is described in which the damper portion 35 according to the embodiment is provided in a displacer-type refrigerator. However, certain embodiments of the invention maybe also applied to a pulse tube-type refrigerator. Hereinafter, a case where certain embodiments of the invention are applied to the pulse tube-type refrigerator will be described.

FIG. 6 is a diagram schematically showing a pulse tube-type cryogenic refrigerator 101. As shown in FIG. 6, the pulse tube-type cryogenic refrigerator 101 includes a first regenerator 102, a second regenerator 103, a first pulse tube 104, and a second pulse tube 105. The high temperature end of each of the first regenerator 102, the first pulse tube 104, and the second pulse tube 105 is connected to a branch pipe 108 which is branched to three segments from a discharge side of a compressor 107 and a branch pipe 109 which is branched to three segments from a suction side, via a first supply-exhaust common pipe 110, a second supply-exhaust common pipe 111, and a third supply-exhaust common pipe 112 corresponding to each high temperature end.

A regenerator supply valve V1 is disposed in front of a first connection point P1 of the branch pipe 108 to the first supply-exhaust common pipe 110. A first supply valve V3 is disposed in front of a second connection point P2 of the branch pipe 108 to the second supply-exhaust common pipe 111. In addition, a second supply valve V5 is disposed in front of a third connection point P3 of the branch pipe 108 to the third supply-exhaust common pipe 112.

A regenerator return valve V2 is disposed in front of the first connection point P1 of the branch pipe 109 to the first supply-exhaust common pipe 110. A first return valve V4 is disposed in front of the second connection point P2 of the branch pipe 109 to the second supply-exhaust common pipe 111. A second supply valve V6 is disposed in front of the third connection point P3 of the branch pipe 109 to the third supply-exhaust common pipe 112.

A flow rate control valve V7 is disposed between the high temperature end of the first pulse tube 104 of the second supply-exhaust common pipe 111 and the second connection point P2. In addition, a flow rate control valve V8 is disposed between the high temperature end of the second pulse tube 105 of the third supply-exhaust common pipe 112 and the third connection point P3. The flow rate control valves function as a phase adjustment mechanism of a gas piston generated in the pulse tube. In addition, instead of the flow rate control valve, an orifice may be used.

A first straightening heat exchanger 113 is disposed on the high temperature end of the first pulse tube 104, and a second straightening heat exchanger 114 is disposed on the low temperature end of the first pulse tube 104. A third straightening heat exchanger 115 is disposed on the high temperature end of the second pulse tube 105, and a fourth straightening heat exchanger 116 is disposed on the low temperature end of the second pulse tube 105.

The low temperature end of the first pulse tube 104 and the low temperature end of the first regenerator 102 are thermally connected by a cooling stage 117. The low temperature end of the first pulse tube 104 and the low temperature end of the first regenerator 102 are connected to each other by a first low temperature end connection pipe 118 positioned inside the cooling stage 117 so that the refrigerant gas circulates therebetween. The low temperature end of the second pulse tube 105 and the low temperature end of the second regenerator 103 are connected to each other by a second low temperature end connection pipe 119 so that the refrigerant gas circulates therebetween.

In addition, similar to the above-described displacer-type second regenerator 34, in the pulse tube-type cryogenic refrigerator 101, the inner portion of the second regenerator 103 includes a high temperature side region 124 having a nonmagnetic material in the upper stage and a low temperature side region 125 having a regenerator material formed of a magnetic material in the lower stage. The high temperature side region 124 and the low temperature side region 125 are combined with each other, and thus, the second regenerator 103 is configured.

In the pulse tube-type cryogenic refrigerator 101 configured in this way, in a process in which the high pressure refrigerant gas is supplied, if the first supply valve V3 and the second supply valve V5 are opened, the refrigerant gas flows into the low temperature ends of the first pulse tube 104 and the second pulse tube 105 via the branch pipe 108 and the second supply-exhaust common pipe 111 or the third supply-exhaust common pipe 112.

Moreover, the regenerator supply valve V1 is opened, the refrigerant gas flows into the low temperature end of the first pulse tube 104 from the first regenerator 102 through the branch pipe 108 and the first supply-exhaust common pipe 110 from the compressor 107, and flows into the high temperature end of the second pulse tube 105 through the second regenerator 103.

Meanwhile, in a process in which a low pressure refrigerant gas is returned, if the first return valve V4 or the second return valve V6 is opened, the refrigerant gas in the first pulse tube 104 or the second pulse tube 105 is returned to the compressor 107 through the second supply-exhaust common pipe 111 or the third supply-exhaust common pipe 112 and the branch pipe 109 from the respective high temperature ends. In addition, if the regenerator return valve V2 is opened, the refrigerant gas in the first pulse tube 104 is returned to the compressor 107 via the first regenerator 102, the first supply-exhaust common pipe 110, and the branch pipe 109 from the low temperature end. Similarly, the refrigerant gas in the second pulse tube 105 is returned to the compressor 107 via the second regenerator 103, the first regenerator 102, the first supply-exhaust common pipe 110, and the branch pipe 109.

In the pulse tube-type cryogenic refrigerator 101, by repeating the operation in which the refrigerant gas such as the helium gas which is compressed by the compressor 107 and is an operating fluid flows into the first regenerator 102, the second regenerator 103, the first pulse tube 104, and the second pulse tube 105, and the operation in which the operating fluid flows from the first pulse tube 104, the second pulse tube 105, the first regenerator 102, and the second regenerator 103, and is returned to the compressor 107, the cooling is formed on the low temperature ends of the regenerator and the pulse tube. In addition, the cooling object comes into thermal contact with the low temperature ends, and thus, it is possible to take heat from the cooling object.

The pulse tube-type cryogenic refrigerator 101 includes a damper portion which is adjacent to the low temperature side region 125 of the second regenerator 103 and communicates with the low temperature side region 125 of the second regenerator 103. The effects are similar to those of the displacer-type cryogenic refrigerator 1. In addition, FIG. 6 shows the case where the damper portion 35 is added to the lower portion of the low temperature side region 125 of the second regenerator 103 in the pulse tube-type cryogenic refrigerator 101. More specifically, in the second regenerator 103, the damper portion 35 is added further downward than the location connected to the second low temperature end connection pipe 119. The damper portion 35 is a container having a short pipe shape in which one end is opened and the other end is closed, the one end is directed in the direction adjacent to the low temperature side region 125 of the second regenerator 103, and the other end is closed by the bottom portion of the container. The damper portion 35 may be a space in which the regenerator material provided in the lower temperature side end portion of the cylinder accommodating the second regenerator 103 is not disposed.

In addition, FIG. 6 shows the case where the second low temperature end connection pipe 119 is provided at the boundary between the low temperature side region 125 of the second regenerator 103 and the damper portion 35. However, the connection location of the second low temperature end connection pipe 119 is not limited to this. The second low temperature end connection pipe 119 may be provided at any region as long as the connection pipe is positioned at the region of the damper portion 35 side rather than the low temperature side region 125.

As described above, according to the cryogenic refrigerator having the damper portion 35 of the certain embodiment of the invention, it is possible to decrease the temperature amplitude while maintaining the average temperature of the generated cooling.

Hereinbefore, the preferred embodiment of certain embodiments of the invention is described in detail. However, certain embodiments of the invention are not limited to the above-described embodiment, and various modifications and replacements may be applied to the above-described embodiment without departing from the ranges of the certain embodiments of the invention.

For example, in the above-described cryogenic refrigerator, the case where the number of stages is two is shown. However, the number of stages may be three stages or more. In addition, in the embodiment, the case where the cryogenic refrigerator is the displacer-type GM refrigerator or the pulse tube-type is described. However, certain embodiments of the invention are not limited to this. For example, certain embodiments of the invention may be applied to a Stirling refrigerator, a Solvay refrigerator, or the like.

It should be understood that the invention is not limited to the above-described embodiment, but may be modified into various forms on the basis of the spirit of the invention. Additionally, the modifications are included in the scope of the invention. 

What is claimed is:
 1. A cryogenic refrigerator comprising: a displacer which includes a low temperature end and a high temperature end; a cylinder which accommodates the displacer to be reciprocated in a longitudinal direction and forms an expansion space of a refrigerant gas between the low temperature end of the displacer and the cylinder; and a damper portion which is disposed to be adjacent to the expansion space and communicates with the expansion space, wherein a volume of the damper portion is 30% or less of a volume of the expansion space.
 2. The cryogenic refrigerator according to claim 1, wherein the damper portion is included in the displacer.
 3. The cryogenic refrigerator according to claim 2, wherein the displacer includes a cover portion at the low temperature end, and wherein the damper portion has a short pipe shape in which one end is opened and the other end is closed, the one end is directed to a side adjacent to the expansion space, and the other end is closed by the cover portion.
 4. The cryogenic refrigerator according to claim 1, wherein the damper portion is provided on the cylinder.
 5. The cryogenic refrigerator according to claim 4, wherein the cylinder includes a flanged cooling stage having a protruding portion which extends in a direction perpendicular to a long axis of the cylinder, at a bottom portion and a position corresponding to the expansion space on an outer periphery, and wherein the damper portion is provided in the protruding portion of the cooling stage.
 6. The cryogenic refrigerator according to claim 1, wherein the cryogenic refrigerator is a multistage cryogenic refrigerator which includes a plurality of the displacers, and wherein the damper portion is installed to be adjacent to the expansion space which is formed between the lowest temperature side displacer and the cylinder. 